An ignition device for an internal combustion engine, having a step-up transformer having a primary winding and a secondary winding, an oscillator connected to the primary winding, and an ignition plug connected to the secondary winding is known (see PTL 1 specified below). When a primary voltage is applied to the primary winding using the oscillator, a secondary voltage is generated at the secondary winding. According to this ignition device, as described later, a high secondary voltage is generated by making use of the resonance phenomenon caused by the leakage inductance of the secondary winding and the stray capacitance parasitic to the leakage inductance. Using this high secondary voltage, electric discharge is generated by the spark plug.
The step-up transformer includes a core made of a soft magnetic material. As described later, the core is provided with a gap for purposes such as making the self-resonant frequency of the secondary winding higher. However, due to the gap, when the step-up transformer is driven, there tends to be problems such as the magnetic flux leaks from the gap, the resonance gain of the secondary voltage decreases, and electromagnetic noise occurs.
Thus, in recent years, attempts have been made to shield the leakage magnetic flux generated from the gap by providing a shielding part made of a conductive material. This configuration intends to thereby suppress electromagnetic noise. In addition, when the leakage magnetic flux is blocked by the shielding part, an induced voltage is generated in the shielding part and a current flows, resulting in the generation of magnetic flux (hereinafter also referred to as induced magnetic flux). Since a part of the induced magnetic flux returns to the core, it can be considered that the resonance gain of the secondary voltage can be improved.